cnic: Wait for all Context IDs to be deleted before sending FCOE_DESTROY_FUNC

Otherwise, the firmware will not respond and we'll have to wait for
timeout.  Refactor the wait loop we already have into a separate
function for this purpose.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Bhanu Prakash Gollapudi <bprakash@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>